A muscle fiber, also spelled muscle fibre (see spelling differences), also technically known as a myocyte, is a single cell of a muscle. Muscle fibers contain many myofibrils, the contractile unit of muscles. Muscle fibers are very long; a single fiber can reach a length of 30cm.

Type I muscle fibers (slow-oxidative fibers) use primarily cellular respiration and, as a result, have relatively high endurance. To support their high-oxidative metabolism, these muscle fibers typically have lots of mitochondria and myoglobin, and thus appear red or what is typically termed "dark" meat in poultry. Type I muscle fibers are typically found in muscles of animals that require endurance, such as chicken leg muscles or the wing muscles of migrating birds (e.g., geese).

Type II muscle fibers use primarily anaerobic metabolism and have relatively low endurance. These muscle fibers are typically used during tasks requiring short bursts of strength, such as sprints or weightlifting. Type II muscle fibers cannot sustain contractions for significant lengths of time, and are typically found in the white meat (e.g., the breast) of chicken.

There are two sub-classes of type II muscle fibers, type IIa (Fast-Oxidative) and IIb (Fast-Glycolytic). The Type IIa fast-oxidative fibers actually also appear red, due to their high content of myoglobin and mitochondria. Type IIb (Fast-Glycolytic) tire the fastest, and are the prevalent type in sedentary individuals. These fibers appear white histologically, due to their low oxidative demand, manifested by the lack of myoglobulin and mitochondria (relative to the Type I and Type IIa fibers). Some research suggests that these subtypes can switch with training to some degree. The biochemical difference between the three types of muscle fibers is their myosin heavy chains.

These are also called the "fast-twitch fibers". They rely primarily on glycolisis (anaerobic metabolism) because it is a fast energy pathway. This type of muscle fiber has the highest rate of contraction of all the muscle fiber types, and therefore can produce the greatest kinetic energy. However, Type IIb muscle fibers suffer the highest rate of fatigue.